In general, molded catalysts or supported catalysts for industrial use should have a mechanical strength above a certain level so as not to be pulverized or disintegrated during their transportation or packing into a reactor. The mechanical strength of the catalysts can be improved to a certain extent by controlling the molding pressure or devising a molding or supporting procedure. The catalysts having a mechanical strength increased by such a method, however, are generally disadvantageous in that the yield of a desired product is low for reasons such as the decreased specific surface area of the catalysts, the decreased number of active sites effective for reaction, and the uncontrollableness of the pore distribution effective for reaction.
For the above purpose, several methods have been proposed. For example, Japanese Patent Unexamined Publication No. 57-119837 has proposed a process for obtaining a catalyst having a certain measure of mechanical strength and capable of giving a high yield of a desired product, by adding an organic high-molecular weight compound such as a cellulose, a polyvinyl alcohol or a polyethylene glycol at the time of molding a catalyst for oxidation of olefins. In this patent, there is used a method which comprises mixing a precursor of catalyst with said organic high-molecular weight compound, molding the mixture, and calcining the resulting molded product in an oxygen-containing atmosphere at a temperature of 400.degree.-700.degree. C., preferably 500.degree.-650.degree. C. to remove said organic high-molecular weight compound. The organic high-molecular weight compounds have such a low depolymerizability that they are hardly decomposed at about 400.degree. C. Therefore, in order not to employ a very high temperature and a very long time for removal, the organic high-molecular weight compound should be removed by combustion in an oxygen-containing atmosphere at a high temperature. In this case, it is appreciated that the catalyst is deactivated by the generation of heat by the combustion, and hence a very difficult procedure is required. In addition, in said patent, there is employed a method which comprises removing said organic high-molecular weight compound added to the catalyst molded product, by calcining, packing the resulting molded catalyst into a reactor, and carrying out a reaction. In order that the molded catalyst freed of said organic high-molecular weight compound may have a mechanical strength enough to withstand the packing, it is necessary to set the molding pressure at the time of molding at a fairly high value. In general, when the mechanical strength of a molded catalyst or a supported catalyst is increased by raising the molding pressure or the supporting pressure, the yield of a desired product is generally often lowered for reasons such as the decreased specific surface area of the catalyst, the decreased number of active sites effective for reaction, and the uncontrollableness of the pore distribution effective for reaction. Therefore, it is not desirable.
The specification of British Patent Laid-Open No. 2138694, which is equivalent to U.S. Pat. No. 4,564,607, has reported a method for increasing the mechanical strength by using a whisker as a support-reinforcing agent in the production of a supported catalyst. However, also in the method of this patent, a carrier itself is required to have a mechanical strength enough to withstand transportation, packing, etc. Therefore, it is necessary to set the supporting pressure at the time of supporting at a fairly high value. Consequently, said method is disadvantageous in that the yield of a desired product tends to be lowered.
On the other hand, the present invention is a molded catalyst or a supported catalyst in which the surface of at least a portion of the molded catalyst or the supported catalyst has been coated with one or more highly depolymerizable organic high-molecular weight compounds in an amount of 0.1 to 40% by weight based on the weight of said catalyst. Furthermore, the present invention is a process which comprises packing the coated catalyst into a reactor, removing said organic high-molecular weight compound(s) by depolymerization before the initiation of a reaction, and then carrying out the reaction. The present invention can be said to be substantially different from the above patents in that in the present invention, it is not necessary to increase the molding pressure or the supporting pressure at the time of molding or the time of supporting unnecessarily.